Since FF does NOT support scripting, neither KM nor AppleScript nor JavaScript in Browser has direct access to the web page.
So it is unlikely that you can automate this using FF.
The best solution is to use Safari or Chrome for this task if you can.
Otherwise, the one possibility is that if it is a simple form that you can move between fields using TAB, then you MIGHT be able to click in the first field, and have KM paste or type the data, TAB to the next field, and paste/type, etc.
The TAB would be done by Type a Keystroke.
Can't say much more without seeing a screen shot of the web form.
Please read:
Tip: How Do I Get The Best Answer in the Shortest Time?